Image processing apparatus

ABSTRACT

A density distribution calculation section  201  calculates the density distribution of a document on the basis of input color image data and a background density level calculation section  202  calculates the background density level of the document on the basis of the calculated density distribution. A density conversion table preparing section  203  prepares a density conversion table on the basis of the background density level. An image conversion section  204  converts a density near a document background density of input color image data on the basis of the density conversion table. Even in the case of a document having a background color, it is possible to suppress a “back page” emergence while reserving the background color and, at the same time, reduce an uneven shade of background density.

TECHNICAL FIELD

The present invention relates to an image processing apparatus forprocessing a color image read out from a document on an image processingapparatus such as a digital type color copier for copying a color imageand to an image forming apparatus for forming a color image with the useof this image processing apparatus.

BACKGROUND ART

Generally, various documents are used as an printing medium and thereare sometimes the situations in which it is not necessary to faithfullycopy a document in accordance with the use to which a copied image isput. In the case of a document, such as a newspaper and notebook, havinga higher background tone, if such a background is faithfully reproduced,then characters in the background becomes lower in contrast and veryillegible.

If the paper sheet is thinner in the magazine, etc., an image of a backpage is often slightly visible on a front page side and, if an image iscopied from the magazine page, then it is copied together with the imageon the back page, so that a “back page” emergence occurs.

In the case where a copy is made from the newspaper, notebook, magazineand so on, copying is made with a thinner density tone set by a manualadjustment, so that the background tone and the “back page” emergenceare not prominent.

The setting of the density as set out above is sometimes doneautomatically. In JPN PAT APPLN KOKAI PUBLICATION NO. 3-88569, thedensity adjustment level is automatically set by detecting the densitydistribution characteristic of an image as a whole and setting thedensity conversion characteristic with the density distributioncharacteristic as a parameter. By doing so, it is not necessary for theuser to set the density level while considering the background tone ofeach document page. And copying is simply carried out.

In the case where the document, such as the newspaper, having thebackground tone is copied with the copying density adjusted to a lowdensity level, the background tone becomes thinner in level but, at thesame time, the characters also become thinner in density level and doesnot necessarily become legible and does not always look clean and clear.Further, the same thing is also true of a document whose back page issomewhat visible from the front page side.

Further, the same situation is again encountered in the case where thesetting of the density level is automatically done as set out above.Indeed, it is not necessary for the user in the automatic setting methodto set the density level with the background tone in mind and a readiercopying operation can be carried out, but not only the background tonebut also characters in the background becomes thinner.

In the case where color copying is carried out, another problem occurs.A color document is usually often given a background color toneintentionally and it is not always desirable to eliminate the backgroundcolor. A distinction should be made between a document, such as thenewspaper, whose background tone should be eliminated and a printedcolor document whose background tone should not be eliminated. For theformer case only, the background should be eliminated.

For the document including a photograph, if the elimination of thebackground tone and that of a “back image” emergence is effected all ata time over a whole image, then the photograph section needs to have itsdensity level to be faithfully reproduced but the density level of thephotograph will be lowered. That is, it is necessary to prevent abackground tone from being eliminated, and a “back image” emergence frombeing so processed, in the photograph section.

In the above-mentioned JPN PAT APPLN KOKAI NO. 3-88569, a “character”section and “photograph” section are identified on a pixel-by-pixelfashion and a corresponding density-level conversion is done with theirsuitable density-level conversion characteristic. By this method it ispossible to properly effect the density-level conversion of the“photograph” section. At the “character” section included in thebackground, the background color tone will always be eliminated even ifa given document is not desirable to have its background toneeliminated.

In the case where a color document, being prominent in a “back image”emergence in particular, has its emergence level lowered by lowering itsbackground tone, some effect is gained if it has not any backgroundcolor tone. If, on the other hand, the color document having abackground color tone is copied, a aback image, emergence is lowered butthe background color tone will change, thus presenting a problem.Therefore, if this is the case, then a countermeasure is to eliminatethe “back image” emergence while reserving the background color tone.

Further, in the case of a color document having a background color tonein particular and having a greater background area over a document wholesurface, uneven shades of density become prominent with the recordingcharacteristics of an ordinary recording apparatus. If, in a digitalcopier for example, the background density is output at a constant levelby detecting the background area, it is possible to suppress even theuneven shades of density.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an image processingapparatus which, in the case of copying a document of a backgrounddensity, lightens a background density and reserves a character densityand, in the case of copying a document involving a “back image”emergence, lightens the background image and reserves a “surface” imagedensity.

Another object of the present invention is to provide an imageprocessing apparatus and image forming apparatus which, even in the caseof a document additionally containing a photograph, converts abackground density of a character area to another value and faithfullyreserves the density of a photograph section and involves no change incolor and in density.

Still another object of the present invention is to provide an imageprocessing apparatus and image forming apparatus which, even if adocument has a background color tone, can suppress a “back image”emergence while reserving the background color tone and reduce an unevenshade of background density.

In order to achieve the above-mentioned object, the image processingapparatus of the present invention comprises density distributioncalculation means for calculating a density distribution of a documentimage on the basis of input document image density data, density rangecalculation means for calculating a density range corresponding to abackground density of the document image on the basis of the densitydistribution calculated by the density distribution calculating means,and conversion means for converting the document image density containedin the background density range calculated by the density rangecalculating means to another density value and outputting it.

The density distribution calculation means has a histogram preparingmeans for preparing a density histogram representing color features ofthe document on the basis of the input image data, and the density rangecalculation means has means for deciding, as a background density levelof the document, a density having a greatest frequency in a low densityarea of the histogram prepared by the histogram preparing means andcalculating the background density range on the basis of the backgrounddensity level.

The conversion means has means for converting, to a value “0”, inputimage data below the background density level calculated by the densityrange calculation means.

The conversion means also has means for converting, to a value “0”,input image data below the lower background density level calculated bythe density range calculation means and converting input image datagreater than the background density level in accordance with apredetermined function.

Further, the conversion means has means for converting, to apredetermined value, the input image data below the background densitylevel calculated by the density range calculation means.

Still further, the conversion means has means for converting, to apredetermined value, only input image data contained in a predetermineddensity range containing the background density level calculated by thedensity range calculation means and outputting the other input imagedata directly.

Further, the density range calculation means has means for deciding, asbeing the background density range, a density range near the backgrounddensity level having a frequency down to a frequency smaller by apredetermined value than a frequency of the background density levelrelative to the image data.

The apparatus according to the present invention is characterized byfurther comprising means for setting the document as being a colordocument or monochrome document and that the conversion means effectsfirst background density conversion with respect to the document set asbeing a color document and effects second background density conversionset as being a monochrome document, the second background densityconversion differing from the first background density conversion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view diagrammatically showing an inner structure of animage forming apparatus according to an embodiment of the presentinvention;

FIG. 2 is a block diagram showing an electrical arrangement of an imageforming apparatus shown in FIG. 1;

FIGS. 3A and 3B are a block diagram showing a major section of an imageprocessing apparatus according to a first embodiment of the presentinvention;

FIG. 4 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to a first embodiment;

FIG. 5 is a block diagram showing an arrangement of a multi-valueobtaining section in an impurity distribution calculation section;

FIG. 6 is a block diagram showing an arrangement of a histogrampreparing section in a density distribution calculation section;

FIGS. 7A to 7C show an example of a histogram;

FIG. 8 is a view showing an example of a histogram;

FIG. 9 is a block diagram showing an arrangement of a background densitylevel calculation section;

FIGS. 10A and 10B show one example of a density conversion table;

FIG. 11 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to a second embodiment;

FIG. 12 is a block diagram showing an arrangement of a backgrounddensity distribution calculation section;

FIG. 13 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to a third embodiment;

FIGS. 14A and 14B are a block diagram showing an arrangement of abackground density conversion section;

FIG. 15 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to a fourth embodiment;

FIG. 16 is a block diagram showing a practical form of an unevenbackground density suppression section;

FIG. 17 is a view for explaining a density conversion effected by theuneven background density suppression section of FIG. 16;

FIG. 18 is a block diagram showing a second practical form of the unevenbackground density suppression section;

FIG. 19 is a view for explaining density conversion effected by theuneven background density suppression section;

FIG. 20 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to a fifth embodiment;

FIG. 21 is a view showing an example of a density distribution;

FIG. 22 is a view for explaining a density conversion;

FIGS. 23A and 23B are an example of a density distribution;

FIG. 24 is a block diagram diagrammatically showing an arrangement of amajor section of an image processing apparatus according to a sixthembodiment;

FIG. 25 is a block diagram showing an arrangement of a backgroundpresence/absence decision section;

FIG. 26 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to a seventh embodiment;

FIG. 27 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to an eighth embodiment;

FIG. 28 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to a ninth embodiment;

FIG. 29 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to a tenth embodiment;

FIG. 30 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to an eleventh embodiment; and

FIG. 31 is a block diagram showing an arrangement of a major section ofan image processing apparatus according to a twelfth embodiment.

BEST MODE OF CARRYING OUT THE INVENTION

The embodiments of the present invention will be explained below withreference to the accompanying drawing.

FIG. 1 is a view diagrammatically showing an inner arrangement of animage forming apparatus, such as a digital type color copier, whichcopies an image from a color image in accordance with the presentinvention. The image forming apparatus is generally separated into acolor scanner section 1 as an image reading means for reading out acolor image on a document and a 4-tandem type color printer as an imageforming means for copying an image from a read-out color image.

The color scanner section 1 has a document glass of transparent glasshaving a document glass cover 3 thereon, arranged opposite to thedocument glass cover in a closed state and allowing a document to be setthereon. Below the document glass are arranged an exposure lamp 5 forilluminating the document placed on the document glass 4, a reflectorfor allowing light which comes from the exposure lamp 5 to be condensedonto the document, a first mirror 7 for allowing the light which isreflected from the document to be bent in a leftward direction relativeto the drawing, and so on. The exposure lamp 5, reflector 6 and firstmirror 7 are fixed to a first carriage 8. The first carriage 8 is drivenby a pulse motor not shown through a toothed belt not shown and, bydoing so, moved parallel to, and along the lower surface of, thedocument glass 4.

On a side to the left of the first carriage 8 in the Figure, that is, ina direction in which the light which is reflected by the first mirror 7is guided, a second carriage 9 is provided which is movable parallel tothe document glass 4 through a drive mechanism (for example, a toothedbelt and DC motor, etc.) not shown. In the second carriage 9, a secondmirror 11 and third mirror are arranged at a right angle with respect toeach other, the second mirror 11 allowing the light from the documentwhich is guided by the first mirror 7 to be bent in a downward directionin the Figure and the third mirror 12 allowing the light from the secondmirror 11 to be bent in the right direction in the Figure. The secondcarriage 9 is driven by the first carriage 8 and moved at a ½ speedalong the document glass 4 relative to the first carriage 8.

In a plane including an optical axis of the light reflected by thesecond and third mirrors 11 and 12, an image formation lens 13 isarranged which allows the light which is reflected from the third mirror12 to be imaged at a predetermined magnification. In a planesubstantially perpendicular to the optical axis of the light passingthrough the image formation lens 13, a CCD type color image sensor(photoelectric conversion device) 15 is arranged which converts thereflected light which is given a focussing to electric signals.

When the light from the exposure lamp 5 is condensed at a document onthe document glass 4 by the reflector 6, the reflected light from thedocument is incident onto the color image sensor 15 through the firstmirror 7, mirror 11, third mirror 12 and color image sensor 15. Thecolor image sensor 15 converts the incident light to the electricsignals corresponding to the primary colors R (red), G (green) and B(blue) of light.

A color printer section 2 has first to fourth image forming sections 10y, 10 m, 10 c and 10 k forming those images subjected to colorseparation for respective color components on the basis of the knownsubtractive mixture, that is, those images of the four colors yellow(Y), magenta (M), cyan (C) and black (K).

Below the respective image forming sections 10 y, 10 m, 10 c and 10 k aconveying mechanism 20 is provided which includes a belt conveyor 21serving as a conveying means for conveying the images for the respectivecolors which are formed by the respective image forming sections to beconveyed in a direction of an arrow a. A belt conveyor 21 is stretchedbetween a drive roller 91 rotated in the direction of an arrow a by amotor not shown and a driven roller 92 spaced a predetermined distancefrom the drive roller 91 and around these rollers 91 and 92 and isendlessly run at a constant speed in the direction of the arrow a. It isto be noted that these image forming sections 10 y, 10 m, 10 c and 10 kare arranged in a serial array along the conveying direction of the beltconveyor 21.

The image forming sections 10 y, 10 m, 10 c and 10 k includephotosensitive drums 61 y, 61 m, 61 c and 61 k, respectively, serving asimage carriers having outer peripheries and rotatable in the samedirection in those positions contacting with the belt conveyor 21.

The respective photosensitive drums 61 y, 61 m, 61 c and 61 k arerotated by a motor, not shown, at predetermined peripheral speeds.

The respective photosensitive drums 61 y, 61 m, 61 c and 61 k have theiraxes arranged at an equal interval and arranged in a directionperpendicular to that in which the images are conveyed by the beltconveyor 21. In an explanation as will be set forth, the axial directionof the respective photosensitive drums 61 y, 61 m, 61 c and 61 k isreferred to as a main scanning direction (second direction) and therotation direction of the photosensitive drums 61 y, 61 m, 61 c and 61k, that is, the rotation direction (direction of an arrow a in theFigure) of the belt conveyor 21, is referred to as a sub-scanningdirection (first direction).

Around the photosensitive drums 61 y, 61 m, 61 c and 61 k, chargers 62y, 62 m, 62 c and 62 k serving as a charging means extending in the mainscanning direction and serving as a charging means, dischargers 63 y, 63m, 63 c and 63 k, developing rollers 64 y, 64 m, 64 c and 64 k servingas developing means similarly extending in the main scanning direction,lower stirring rollers 67 y, 67 m, 67 c and 67 k, upper stirring rollers68 y, 68 m, 68 c and 68k, transfer units 93 y, 93 m, 93 c and 93 kserving as transfer means similarly extending in the main scanningdirection, cleaning blades 65 y, 65 m, 65 c and 65 k similarly extendingin the main direction and spent toner recovery screws 66 y, 66 m, 66 cand 66 k are sequentially arranged along the rotation direction of thephotosensitive drums 61 y, 61 m, 61 c and 61 k.

The respective transfer units 93 y, 93 m, 93 c and 93 k are arrangedrelative to the photosensitive drums 61 y, 61 m, 61 c and 61 k with thebelt conveyor 21 placed therebetween, that is, arranged on an inner sideof the belt conveyor 21. The exposure point by a later describedexposure device 50 is formed on an outer peripheral surfaces of thephotosensitive drums 61 y, 61 m, 61 c and 61 k between the chargers 62y, 62 m, 62 c and 62 k and developing rollers 64 y, 64 m, 64 c and 64 k,respectively.

Below the conveying mechanism 20, sheet cassettes 22 a, 22 b arearranged to store a plurality of sheets P as an image forming medium towhich images formed by the image forming sections 10 y, 10 m, 10 c and10 k are transferred.

One end sides of the sheet cassettes 22 a, 22 b and on a side near to adriven roller 92, pickup rollers 23 a, 23 b are arranged to allow sheetsP which are stored in the sheet cassettes 22 a, 22 b to be picked up oneby one from their top. Between the pickup rollers 23 a, 23 b and thedriven roller 92 a register roller 24 is arranged to match the forwardend of the sheet P which is picked up from the sheet cassettes 22 a, 22b to the forward end of a Y toner image formed on the photosensitivedrum 61 y of the image forming section 10 y.

The toner images formed on the remaining photosensitive drums 61 y, 61 mand 61 c are supplied to the respective transfer positions at aconveying timing of the sheet P conveyed on the conveying belt 21.

Between the register roller 24 and the first image forming section 10 yand in the neighborhood of the driven roller 92, that is, on the outerperiphery of the driven roller 92 with the belt conveyor 21substantially sandwiched, an attraction roller 26 is arranged to applyan electrostatic attraction to the sheet P conveyed at a predeterminedtiming through the register roller 24. The axis of the attraction roller26 and that of the driven roller 92 are set parallel to be each other.

On one end side of the conveying belt 21 and in the neighborhood of thedrive roller 91, that is, on the outer periphery of the drive roller 91with the belt conveyor 21 substantially sandwiched, a positionaldisplacement sensor 96 is arranged to detect the position of an imageformed on the belt conveyor 21. The positional displacement sensor 96 iscomprised of a light transmitting or a light reflecting type lightsensor.

At the outer periphery of the drive roller 91 and at the belt conveyor21 on the downstream side of the positional displacement sensor 96 abelt conveyor cleaning device 95 is arranged to remove a toner depositedon the belt conveyor 21 or a paper dust, etc., of a sheet P.

In a direction further conveying the sheet P after the sheet P which isconveyed through the belt conveyor 21 has been released from the driveroller 91, a fixing device 80 is arranged to melt a toner image which istransferred to the sheet P, by heating the sheet P to a predeterminedtemperature, and fix the toner image to the sheet P. The fixing device80 comprises a heat roller pair 81, oil coating rollers 82, 83, webwind-up roller 84, web roller 85 and web pressing roller. The tonerformed on the sheet P is fixed to the sheet and discharged out of asheet discharge roller pair 87.

The exposure device 50 forming color-separated electrostatic latentimage on the outer peripheral surfaces of the photosensitive drums 61 y,62 m, 61 c and 61 k has a semiconductor layer oscillator 60 subjected tolight emission control based on image data (Y, M, C, K) for respectivecolors which are color-separated by a later described image processingdevice 36. On an optical path of a semiconductor laser oscillator 60, apolygon mirror 51 rotated by a polygon motor 54 to reflect and scan alaser beam and fθ lenses 52, 53 allowing the focussing of the laser beamwhich is reflected through the polygon mirror 51 to be corrected andimaged are arranged in a sequential way.

Between the fθ lens 53 and the photosensitive drums 61 y, 61 m, 61 c and61 k, first bending mirrors 55 y, 55 m, 55 c and 55 k allowing laserbeams of respective colors which pass through the fθ lens 53 to be benttoward the exposure positions of the respective photosensitive drums 61y, 61 m, 61 c and 61 k and second and third bending mirrors 56 y, 56 m,56 c, 57 y, 57 m and 57 c allowing the laser beams which are bent by thefirst bending mirrors 55 y, 55 m, 55 c to be further bent are arranged.

The laser beam for black is bent by the first bending mirror 55 k andthen guided onto the photosensitive drum 61 k without passing throughother mirrors.

FIG. 2 is a block diagram schematically showing an electrical connectionof the digital copier as shown in FIG. 1 as well as a flow of signalsfor control. In FIG. 2, a control system comprises a main CPU (centralprocessing unit) 91 in a main control section 30, a scanner CPU 100 inthe color scanner section 1 and a printer CUP 110 in the color printer2.

The main CPU 91 conducts an interactive communication through theprinter CPU 110 and sharing RAM (random access memory) 35. The main CPU91 issues operation instructions and the printer CPU 110 returns acondition status. A serial communication is conducted between theprinter CPU 110 and the scanner CPU 100, the printer CPU 110 issuesoperation instructions and the scanner CPU 100 returns a conditionstatus.

The operation panel 40 has a liquid crystal display 42, various kinds ofoperation keys 43 and panel CPU 41 connected to these and is connectedto the main CPU 91.

The main control section 30 comprises a main CPU 91, ROM (read onlymemory), 32, RAM 33, NVRAM 34, sharing RAM 35, image processing device36, page memory control section 37, page memory 38, printer controller39 and printer font ROM 21.

The main CPU 91 performs general control. The ROM 32 stores a controlprogram, etc. The RAM 33 temporarily stores the data.

NVRAM (nonvolatile random access memory) 34 is comprised of anonvolatile memory backed up by a battery, not shown and retains memorydata even when a power supply is cut off.

The sharing RAM 35 is used to conduct an interactive communicationbetween the main CPU 91 and the printer CPU 110.

The page memory control section 37 enables the page memory 38 to storeimage information and read out it. The page memory 38 has an areacapable of storing image information corresponding to a plurality ofpages and is so structured that compressed data of image informationfrom the color scanner section 1 can be stored per page.

The printer font ROM 121 stores font data corresponding to the printdata. The printer controller 39 enables the print data from an externaldevice 122, such as a personal computer, to be developed to image datawith a resolution corresponding to data representing a resolutionimparted to the print data and to do so with the use of font data storedin the printer font ROM 121.

The color scanner section 1 comprises the scanner CPU 100 for performinggeneral control, ROM 101 storing control program, etc., RAM 102 for datastorage, CCD driver 103 driving the color image sensor 15, scanningmotor driver 104 controlling the rotation of the scanning motor formoving the first carriage 8, etc., and image correction section 105.

The image correction section 105 comprises an A/D conversion circuitconverting R, G and B analog signals which are output from the colorimage sensor 15 to respective digital signals, shading correctioncircuit correcting a variation of a threshold level against an outputsignal from the color image sensor 15 caused by a variation of the colorimage sensor 15 or by a surrounding temperature variation, etc., linememory once storing a shading-corrected digital signal from the shadingcorrection circuit, and so on.

The color printer section 2 comprises the printer CPU 110 performinggeneral control, ROM 111 storing a control program, etc., RAM 112 fordata storage, laser driver 113 driving the semiconductor laseroscillator 60, polygon motor driver 114 driving the polygon motor 54 ofthe exposure device 50, conveyance control section 115 controlling theconveying of the sheet P by the conveying mechanism 20, process controlsection 116 controlling the charging, developing and transfer processesby the use of the charger, developing roller and transfer device, fixingcontrol section 117 controlling the fixing device 80, option controlsection 118 controlling options, and so on.

The image processing device 36, page memory 38, printer controller 39,image correction section 105 and laser driver 113 are connected by animage data bus 120.

FIG. 3 schematically shows an arrangement of an image processing device36. In FIG. 3, color image data R, G, B output from the color scannersection 1 are sent to a position matching interpolation section 151where a position matching interpolation is performed on the color imagedata R, G, B. That is, in general, in the case of the enlarging andscaling of the image read from the document, digital processing isperformed on the image read out in the main scanning direction and themoving speed of the scanner carriage is varied with respect to the imageread out in the sub-scanning direction. In the case where a RGB threeline CCD sensor (8 line pitch) is used as a color image sensor 15, thereoccurs no problem at a 100%/integral multiple magnification. In othermagnifications, a positional displacement occurs, in the sub-scanningdirection, among the R, G, B. The position matching interpolationsection 151 interpolates pixel values on the basis of this displacementamount and, by doing so, corrects the positional displacement.

The color image data R, G, B output from the positional matchinginterpolation section 151 are sent to an ACS 152, monochrome generationsection 153, image processing section 154 and macro identificationsection 155.

The ACS 152 identifies whether the read-out document is a color documentor a monochrome document. It makes their decision at a prescan time andswitching is made, at a main scan time, to either of color processingand monochrome processing.

The monochrome generation section 153 generates a monochrome image datafrom the R, G, B color image data at a monochrome copying mode time. Theimage processing section 154 performs background tone eliminationprocessing on a document having a background tone as will be set forthbelow.

The macro identification section 155 identifies a photograph area andcharacter area in the read-out document. That is, it makes a globaldecision based on a run image input to the page memory through theprescanning of the document. A result of the decision by the macroidentification section 155 is once stored in the identification memory156 and, at the main scanning time, output to the macro identificationsection 160.

The output of the image processing section 154 is sent to the colorconversion section 157. The input signals from the color scanner section1 are those of the R, G, B and the signals at the color printer section2 are those of C, M, Y, K. It is, therefore, necessary to effect theconversion of the color signals. At the color conversion section 157,the image data of R, G, B are converted to the image data of C, M, Y. Acolor adjustment can also be effected, as the user likes, by convertingcolor conversion parameters.

The outputs (color image data C, M, Y) of the color conversion section157 are sent to a lowpass filter (LPF) 158, highpass enhanced filter(HEF) 159 and micro identification section 160.

The lowpass filter 158 and highpass enhanced filter 159 perform spatialfilter processing, such as the noise elimination, moire elimination,edge enhancement, etc. The output of the lowpass filter 158 is sent to acombining section 161 and the output of the highpass enhanced filter 159is sent to a character enhancing section 162.

The micro identification section 160 decides a photograph area andcharacter area in the document. Here, the decision is made withreference to a local area of about 3×3 pixels for example. Based on aresult of this decision it is possible to switch respective processes ofthe combining section 161, character enhancing section 162, black addingsection 169, black character generation section 170, selector 171,record processing section 173 and screen processing section 175.

The character enhancing section 162 performs the character portionenhancing processing and a result of this processing is sent to thecombining section 161. The combining section 161 combines the output ofthe lowpass filter 158 and that of the character enhancing section 162and a result of this combination is sent to a scaling section 163. Thescaling section 163 performs enlarge/reduce processing in the mainscanning direction.

In rotate processing of an electronic source and image, etc., forexample, the image data is once stored in the page memory 38 and therespective processing section performs the respective processing byreading out a portion necessary to an object of processing from the pagememory 38 as the occasion arises. It is, therefore, necessary to readout any area of the image at a given rate. In the case where the imagedata is stored in the page memory 38, first a YIQ conversion section 164and error diffusion section 165 perform compress/expand processing at afixed length.

In the YIQ conversion section 164, the image data of C, M, Y areconverted to Y, I, Q data and the redundancy of color components iseliminated and, in the error diffusion section 165, bit reduction iseffected while reserving the gradation by the error diffusion. At thetime of reading out compressed image data from the page memory 38, theexpansion of the image data and conversion of the Y, I, Q data to the C,M, Y data are carried out in the CMY converter 166.

At the operation time of the electronic sorting function in which thepage memory 38 alone is not adequate for its capacity, the image data isdesigned to be stored in the hard disc device (HDD) 167. At that time,the access speed to the hard disc device 167 is limited and variablelength compress-processing of as good a compression efficiency aspossible is carried out in the variable length compression section 168.

The output of the scaling section 163 is sent to the black addingsection 169 and black character generation section 170 and the blackadding section 169 generates a black signal K from the image data C, M,Y and the black adding processing for adding the black signal K to theimage data C, M, Y is carried out.

The black character generation section 170 additively overlaps the imagedata C, M, Y with one another and generates a black signal K. However,the black character becomes higher in image quality in terms of thecolor and resolution property by making recording with one black colorthan by making recording with the image data C, M, Y overlapped with oneanother. Therefore, the selector 171 switches the output of the blackadding section 169 and output of the black character generation section170 to an identification signal which is output from the microidentification section 160 and is output to a γ correction section 172.

The γ correction section 172 corrects the γ character of the printersection 2. At the time of making this correction, reference is made to γtables set for each of the image data C, M, Y, K.

The output of the γ correction section 172 is sent to the recordprocessing section 173. The record processing section 173 performsgradation processing, such as error diffusion, etc., and the image dataof the input 8 bits is converted to data of about 4 bits withoutimpairing the gradation tone.

For the 4-tandem type image forming apparatus for example, the phaseswith which the four color image signals are recorded are different fromone another and, at a direct memory 174, delays corresponding to therespective phases are given to the respective image signals.

For the 4-tandem type image forming apparatus, even if the image signalsof the respective colors are similarly output with the laser beamoptical system, a moire and color error are generated due to a slightskew of each color, magnification error, etc. In the screen processingsection 175, therefore, with respect to the output of the recordingprocessing section 173, an angle is set to the screen of the respectivecolor, thus suppressing the generation of the moire and color error.

The output of the screen processing section 175 is supplied to a pulsewidth conversion section 176. Since the signal levels subjected by therespective sections to image processing and recording density are notlinear, the pulse width conversion section 176 controls the pulse drivetime of the laser modulation section in the printer section 2 and effectthe pulse width conversion so as to obtain a linear characteristic. Theoutput of the pulse width conversion section is sent to the printersection 2.

FIG. 4 shows an arrangement of a major section of an image processingapparatus according to a first embodiment of the present invention. Forbrevity in an explanation in FIG. 4 et seq., those other than the imageprocessing section 154 are omitted in illustration and will be explainedbelow in this context.

In FIG. 4, the color image data, R, G, B output from the color scannersection 1 is sent to a density distribution calculation section 201through the position matching interpolation section 151. The densitydistribution calculation section 201 calculates a density distributionof the color of the document as color features of the document on thebasis of the input color image data R, G, B and a result of calculationis sent to a background density level calculation section 202. Thebackground density level calculation section 202 calculates thebackground density level of the document on the basis of the densitydistribution calculated at the density distribution calculation section201 and sends a result of calculation to a density conversion tablepreparing section 203. The density conversion table preparing section203 prepares, on the basis of the underground density level calculatedat the underground density level calculation section 202, a densityconversion table for use at the time of conversion at the imageconversion section 204. The image conversion section 204 converts theimage density of the input color image data R, G, B on the basis of thedensity conversion table prepared at the density conversion tablepreparing section 203.

The respective sections will be explained in more detail below.

First, an explanation will be given below about the density distributioncalculation section 201. The density distribution calculation section201 calculates the density distribution of the color of the document andis constituted by a histogram extracting means comprising a multi-valueobtaining section 181 serving as a multi-value obtaining means as shownin FIG. 5 and a histogram preparing section 182 serving as a histogrampreparing means as shown in FIG. 6.

The multi-value obtaining section 181 applies multi-value obtainingprocessing to the input image data R, G, B by comparison withpredetermined threshold values Th1 to Thn−1 and outputs multi-valueimage signals Rg, Gg, Bg. As shown in FIG. 5, it comprises a thresholdvalue memory 183 storing an n−1 number of threshold values Th1 to Thn−1,an n−1 number of comparators 184 ₁ to 184 _(n−1) comparing the inputimage data R (G, B) with the threshold values Th1 to Thn−1, and encoder185 encoding a result of comparisons of the comparators 184 ₁ to 184_(n−1).

Although, in FIG. 5, only a circuit for the image data R is shown, asimilar circuit is also provided for the image data G, B in actualpractice and their illustration is omitted.

Here, the operation of the multi-value obtaining circuit 181 will beexplained below with the number of the multi-valued levels given by n.First, the input image data R (values 0-0255) are compared by thecomparators 184 ₁ to 184 _(n−1) with the threshold values Th1 toTh_(n−1) in the threshold memory 183. The respective comparatorgenerates an output “0” when the input image data is smaller than thethreshold value and an output “1” when otherwise. The encoder 185converts a result of comparisons to a multi-valued version and outputsan image signal Rg. As a result, the multi-value obtaining sectionconverts the input image data R to a multi-valued version as given belowand outputs a multi-valued image signal Rg.

Rg=0:R<Th1

Rg=1:R≧Th1 and R<Th2

Rg=2:R≧Th2 and R<Th3

Rg=3:R≧Th3 and R<Th4

Rg=n−2:R≧Thn−2 and R<Thn−1

Rg=n−1:R≧Thn−1  (1)

Regarding the image data G, B, a similar calculation is made in the sameway as set out in connection with the image data R and multi-valuedimage signals Gg, Bg are calculated.

The histogram preparing section 182 prepares a histogram information onthe basis of the multi-valued image signals Rg, Gg, Bg output from themulti-value obtaining section 181. The histogram preparing section 182comprises, as shown in FIG. 6, a decoder 186 decoding the inputmulti-valued image signal Rg (Gg, Bg), n number of adders 187 ₀, 187 ₁,. . . 187 _(n−1) and n-number of registers 188 ₀, 188 ₁. . . 188 _(n−1).

Although, in FIG. 6, only the circuit for the multi-valued image signalRg is shown, a similar circuit is also provided for the multi-valuedimage signals Gg, Bg in actual practice and their illustration isomitted.

Here, the operation of the histogram preparing section 182 will beexplained below. In the case where the registers 188 ₀ to 188 _(n−1)receiver an image of A3 size and 400 dpi for example, 25 bits arenecessary. The respective registers 188 ₀. . . 188 _(n−1) are allcleared initially to ┌0┘. If the multi-valued image signal Rg is ┌0┘,┌1┘ is added to the adder 187 ₀. The register 188 ₀ retains the outputof the adder 187 ₀ and outputs it to the adder 187 ₀. That is, the adder187 ₀ adds together the output of the register 188 ₀ and that of thedecoder 186.

If the multi-valued image signal Rg is “1”, then “1” is added to theadder 187 ₁ and, if the multi-valued image signal Rg is “2”, then “1” isadded to the adder 187 ₁. As a result, histogram information is preparedin the registers 188 ₀. . . 188 _(n−1). These processes are doneindependently on the multi-valued image signals Rg, Gg and Bg.

The process is repeatedly done for the sequentially input pixel andrepeatedly done until the image inputting of one page is finished.Hereinbelow, the respective frequency (histogram information) of thoseinformation items cumulated from the register 188 ₀ (low densityportion) to the register 188 _(n−1) (high density portion) will beexplained below as RH(0), RH(1), . . . , RH(n−1) for the image data Rand as GH(0), GH(1), . . . , G(n−1) and BH(0), BH(1), . . . , BH(n−1)for the image data G and B.

The histogram preparing section 182 produces histograms as shown in FIG.7. FIG. 7 is a typical example (n=8) of a monochrome document and RH,GH, BH reveal substantially similar frequencies and have greaterfrequencies at the respective high density portion and low densityportion.

The extraction of color features is found based on the mutual values ofR, G, B (that is, not independently of R, G, B) and it is necessary tohave a large quantity of registers as given below:

g=0:R<Th1 and G<Th1 and B<Th1

g=1:R>Th1 and R<Th2 and G<Th1 and B<Th1

g=n:R<Th1 and G<Th1 and G<th2 and B<Th2

g=n ³−1:R≧Thn−1 and G≧Thn−1 and B≧Thn−1  (2)

That is, it is necessary to have an n³ number of registers.

In this embodiment, on the other hand, histogram information is found,by a density distribution calculation section 201, for the image data R,G, B and, by doing so, it is possible to extract the color featuressatisfying the usage of the present invention and to largely reducememories involved. It is only necessary to have a n×3 number ofregisters for the present embodiment.

Next, an explanation will be given below about background density levelcalculation section 202. The background density level calculationsection 202 calculates a background density level (or the level of eachcolor) of the read-out document on the basis of the density distributioninformation calculated by the density distribution calculation section201.

Hereinbelow, the background density level calculation section 202 willbe explained below with the use of an example of reading out amonochrome document of FIG. 8 in a monochrome mode. In the example asshown in FIG. 8, the abscissa denotes the density and the ordinate thefrequency. That is, the low density level corresponding to thebackground is located to the left and the high density levelcorresponding to the character to the right. The background densitylevel calculation section 202 decides a background density level by adeciding equation as given below.

Hmax=max(H(0), H(1), . . . , H(Bmax))  (3)

BL: a density level having an Hmax value  (4)

Here

Hmax: a maximum density distribution value;

BL: a calculated background density level; and

Bmax: a range of a background area

That is, as indicated by the equation (3), the maximum densitydistribution value Hmax is found in a range of Bmax from the density ┌0┘and, as indicated by the equation (4), the density level of the maximumdensity distribution value Hmax is the background density level BL. Inthe example shown in FIG. 8, the density distribution value H(1) ismaximal and the density level at that time, that is, the calculatedbackground density level BL is ┌1┘.

FIG. 9 is one practical form of a circuit implemented with thebackground density level calculation section as a hardware and itcomprises three comparators 301, 302 and 303 and three selectors 304,305 and 306. This is an example of Bmax=3.

The comparator 301 receives a density distribution values H(0) and H(1)and outputs a greater value H(n) and greater select signal SL1. Theselector 304 receives the density levels LV0, LV1 of densitydistribution values (0) and H(1) and a greater density level of thedensity distribution value is selected and output by a select signal SL1output from the comparator 301. In the example of the densitydistribution of FIG. 8, the H(1) as the density distribution value isoutput as a density level.

The comparator 302 and selector 305 operate in a similar fashion and, inthe example shown in FIG. 8 H(2) and ┌2┘ are output as the densitydistribution value and density value, respectively. The comparator 303and selector 306 receive the outputs of the comparators 301, 302 andoutputs of selectors 304, 305, respectively, and receive the outputs ofthe selectors 304, 305 and operate similarly to the comparators 301, 302and selectors 304, 305. In the example of FIG. 8, the H(1) and ┌1┘ areoutput as a maximum density distribution value Hmax and backgrounddensity level BL, respectively.

Next, an explanation will be given below about the density conversiontable preparing section 203. The density conversion table preparingsection 203 prepares a density conversion table on the basis of abackground density level prepared by the background density levelcalculation section 202.

FIGS. 10A and 10B are one example of the density conversion table. Atable (256 byte; 356×3 bytes for a color RGB) for converting an inputsignal of 8 bits (256 levels) is prepared based on the backgrounddensity level BL calculated by the background density level calculationsection 202.

For the case of FIG. 10A, an output Do is ┌0┘ in the case where theinput density level Di is below the background density level BL andDo=Dix(255−BL)/255 in the case where the input density level Di is abovethe background density level BL. Here, a 16-step background densitylevel BL calculated by the background density level calculation section202 is converted to one of 256-step density levels as will be set outbelow.

For the case of FIG. 10B, the output Do is ┌0┘ in the case where theinput density level Di is below the ┌BL┘ and Do=Di in the case where theinput density level Di is above the background density level BL. Ineither case, the output density level is ┌0┘ in the case where the inputdensity level Di is below the background density level BL and it ispossible to remove the background level.

Next, an explanation will be given below about the image conversionsection 204. The image conversion section 204 converts the image densityon the basis of the density conversion table prepared by the densityconversion table preparing section 203. The image conversion section 204is comprised of a RAM (256×3 bytes for the color RGB case) of, forexample, 8 bits×56=256 bytes. It is possible to obtain an output imagedensity by reading out the contents of the RAM with the input imagedensity as an address.

By the above-mentioned arrangement it is possible to remove thebackground density.

Next an explanation will be given below about the second embodiment.

In the first embodiment, the background density level is calculatedwhile, on the other hand, the background density level is calculated inthe second embodiment taking into consideration the broadening of thebackground density distribution.

FIG. 11 shows an arrangement of a major portion of an image processingapparatus 36 according to the second embodiment. The second embodimentis different from the first embodiment in that, in place of thebackground density level calculation section 202, use is made of abackground density distribution calculation section 205. The otherportion of the second embodiment is the same as that of the firstembodiment and any further explanation is omitted with the samereference numerals employed to designate part and element correspondingto those shown in the first embodiment.

FIG. 12 shows a practical form of the background density distributioncalculation section 205 and it comprises three comparators 301, 302,303, three selectors 304, 305 and 306 and adder 307.

The comparator 301 receives density distribution values H(0) and H(1)and outputs a greater H(n) value and greater select signal SL1. Theselector 304 receives the density levels LV0, LV1 of the densitydistribution values H(0) and H(1) and selects and outputs a densitylevel of a greater density value by a select signal SL1 output from thecomparator 301. In the example of the density distribution of FIG. 8,H(1) as a density distribution value is output as a density distributionvalue.

The comparator 302 and selector 305 operate in the same manner and, forthe case of FIG. 8, H(2) and ┌2┘ are output as a density distributionvalue and density level, respectively. The comparator 303 and selector306 receive the respective outputs of the comparators 301, 302 andrespective outputs of selectors 304, 305. The comparators 301, 302 andselectors 304, 305 operate in a similar fashion.

The adder 307 adds a predetermined level to the output of the selector306. In the case of FIG. 8, at L=1, H(1) and ┌2┘ are output as a maximumdensity distribution value Hmax and background density level BL,respectively.

By the above-mentioned arrangement, it is possible to better remove thebackground level even in the case where the background density is unevento some extent.

Then, a third embodiment will be explained below.

In the above-mentioned first and second embodiments, the image densityis converted based on the density conversion table while, in a thirdembodiment, this is done through calculations all with the use ofhardware.

FIG. 13 is an arrangement of a major portion of an image processingapparatus 36 according to a third embodiment. The third embodiment isdifferent from the second embodiment in that the density conversiontable preparing section 203 is eliminated and that, instead of the imageconversion section 204, a background density conversion section 206 isused. The remaining portion is the same as that of the second embodimentand the same reference numerals are employed to designate parts orelements corresponding to those shown in the second embodiment and anyfurther explanation is omitted.

FIGS. 14A and 14B show a practical form of the background densityconversion section 206. In the case of FIG. 14A, it comprises asubtracter 308 for effecting subtraction between an input density levelDi and a background density level BL and subtracter 309 for effectingsubtraction between the output of the subtracter 308 and a predeterminedvalue ┌255┘. It follows that:

input density level Di<background density

level BL: output level Do=0

input density level Di≧background density

level BL: output level Do=Di×(256−BL)/255  (5)

In the case of FIG. 14B, the practical form comprises a comparator 310for comparing the input density level Di and background density level BLand selector 311 for selecting either one of the input density level Dior predetermined value ┌0┘ by a result of comparison of the comparator310. It follows that:

input density level Di<background density

level BL: output level Do=0

input density level Di≧background density

level BL: output level Do=Di  (6)

Next an explanation will be given about a fourth embodiment.

The fourth embodiment is such that, in the case of a color document of acolor background, it is effective to suppress an uneven shade of densityand “back page” emergence, not eliminate the background tone.

FIG. 15 diagrammatically shows an arrangement of a major part of animage processing apparatus according to the fourth embodiment. Thefourth embodiment is different from the third embodiment in that anuneven background density suppression section 207 is used instead of thebackground density conversion section 206. The remaining portion of thefourth embodiment is the same as that of the third embodiment with thesame reference numerals employed to designate part or elementcorresponding to that shown in FIG. 3 and any further explanationomitted.

FIG. 16 shows a first practical structure of the uneven backgrounddensity suppression section 207. It comprises a comparator 312 forcomparing a background density level BL and input density level Di and aselector 313 for selecting either one of the background density level BLor input density level Di.

That is, the uneven background density suppression section 207 effectsdensity conversion as shown in FIG. 17 and, based on the backgrounddensity level BL output from a background density distributioncalculation section 205,

 input density level Di<background density

level BL: output level Do=BL

input density level Di≧background density

level BL: output level Do=Di  (7)

Through this calculation, the image density below the background densityis replaced one at a time by a background density level BL and it ispossible to suppress any uneven background density and “back page”emergence.

FIG. 18 shows a second practical structure of the uneven backgrounddensity suppression section 207. It comprises a subtracter 314 forperforming a subtraction between a background density level BL and agiven level 1, adder 315 for performing an addition between thebackground density level BL and the given level 1, comparator 316 forcomparing the output of the subtracter 314 and an input density levelDi, comparator 317 for comparing the output of the adder 315 and inputdensity level Di, AND circuit 318 for Anding the outputs of thecomparators 316, 317 and a selector 319 for selecting either one of thebackground density level BL or input density level Di by the output ofthe AND circuit.

That is, the uneven background density suppression section 207 is suchthat, in the case of a color document, it is effective to suppress anuneven shade of density and “back page” emergence, not eliminate thebackground in the case of the document of a background tone. Further,the uneven background density suppression section 207 is such that, inthe case of an image thinner in tone than the background (for example, awhite character in the background and a white area of a document settingcover outside the document), it is particularly effective.

FIG. 18 shows a second practical structure of the uneven backgrounddensity suppression section 207 and it performs a density conversionshown in FIG. 19. The second uneven background density suppressionsection 207 performs calculation based on the background density levelBL output from the background density distribution calculation section205:

input density level Di<background density

level BL−1: output level Do=Di

input density level Di≧background density

level BL−1 and Di≦BL+1: output level Do=BL

input density level Di>background density

level BL+1: output level Do=Di  (8)

Through the calculation, the image density near the background densityis replaced one at a time by the background density level BL and it ispossible to suppress an uneven shade of the background tone and “backpage” emergence. Further, any image density clearly thinner in tone thanthe background is reserved.

Next an explanation will be given below about a fifth embodiment. Thefifth embodiment detects the peak position of a background density at animage density distribution and a lower edge position of the densitydistribution showing a spread of the background density and converts thebackground density from the peak position and lower edge position.

FIG. 20 diagrammatically shows an arrangement of a major portion of animage processing apparatus 36 according to a fifth embodiment. Thisembodiment is different from the third embodiment in that a backgroundposition detection section 208 and lower edge position detection section209 are used instead of the background density distribution calculationsection 205. The remaining portion of the fifth embodiment is the sameas that of the third embodiment with the same reference numeralsemployed to designate part or element corresponding to that of the thirdembodiment and any further explanation omitted.

An explanation will be given below about the lower edge positiondetector 209 shown in FIG. 21. The lower edge position is found as aminimal and a maximal density level of a density level H(n) satisfying

H(n)≧Hmax×k  (9)

with respect to a frequency value (that is, a density distributionvalue) continuously decreasing in a monotonic fashion from a peakposition (Hmax) of the background density BL. In an example of FIG. 21,a minimum density level BLmin is ┌0┘ and a maximum density level BLmaxis ┌3┘.

FIG. 22 shows an example of a density conversion based on the lower edgepositions BLmin, BLmax and it is possible to effect density conversionas indicated by a solid line or a dash dot line. In the above-mentionedmethod, density conversion is effected with respect to an input imagedensity included in a density area of a predetermined width with abackground density BL as a center. Since, by this method, the densityconversion is effected in accordance with the width of the uneven shadeof background density, it is possible to effect the image densityconversion with higher accuracy.

Also effective is the method by which offset values are given to thelower edge positions BLmin, BLmax as indicated by the followingequations:

BLmax′=BLmax+Omax

BLmin′=BLmin+Omin  (10)

In the example given above, the background density has its positionalaccuracy determined depending upon the density division number of thedensity distribution calculation section 201. That is, if the divisionnumber is given by n, the positional accuracy becomes.

±256/2n  (11)

That is, if the peak position is given by “p”, the background density BLcorresponding to the image density becomes

BL=(256/n)p+256/2n  (12)

Here, “p” is a density position having a peak frequency on the histogramand, if the density division number is 16, it is one of 0 to 15. “BL” isa background density and, if the resolution of the scanner section 1 is8 bits, it is one value of 0 to 255.

Through the utilization of the density distribution before and after thepeak position, however, it is possible to calculate or correct thebackground density BL more precisely. For example, the frequency H(p) ofthe peak position p is weighted with the frequencies H(p−1) and H(p+1)before/after the peak position and

BL=(256/n)+256/2n+(256/2n)×(H(p+1)−H(p−1))/2H(p)  (13)

And it is possible to correct the background density BL.

In this calculation method, as shown in FIG. 21, if the peak position isover “1”, it is possible to make correction more accurately. If, on theother hand, the peak position is ┌0┘ as shown in FIG. 23A, there arisesa problem because of a lack of any distribution below ┌0┘. Although thefrequency below ┌0┘ is calculated as ┌0┘, the accuracy is poor. In thecase of a color image in particular, for respective the image data R, G,B or C, M, Y, K, the distribution differs between the case of FIG. 21and the case of FIG. 23A. If this is the case, a color channel balanceis broken and any accurate background elimination and further anyaccurate density conversion over a whole image density are not carriedout, thus leaving a background color tone and causing a color hue overthe whole image.

As shown in FIG. 23, in the case of the peak position ┌0┘, the frequencybelow the image density ┌0┘ is virtually prepared and, also with the useof a frequency H(−1) below the image density ┌0┘ it is possible to carryout accurate image density conversion by the calculation of the equation(13).

Next, an explanation will be given below about a sixth embodiment.

The background removal method is done either in a better way or in aworse way depending upon a document as a target. For example, in thecase of a newspaper which is made of an inexpensive paper sheet to makeit at low costs, it is effective to remove the background in thesituation in which a color sheet needs to be used. On the other hand,there is sometimes the case where a color-printed sheet has to be usedintentionally as in the advertisement in which case it is not desirableto remove the background.

In the sixth embodiment, it is considered that decision is made whetheror not a background be removed and, if any given document whosebackground be better to be removed is involved, then it is done so.

FIG. 24 diagrammatically shows an arrangement of a major part of theimage processing apparatus 36 according to a sixth embodiment. Thisembodiment is different from the third embodiment in that a backgroundpresence/absence decision section 210 is added which is adapted todecide whether or not a background be removed. The remaining portion ofthis embodiment is the same as that of the third embodiment with thesame reference employed to designate part or element corresponding tothat of the third embodiment and any further explanation omitted.

FIG. 25 shows an arrangement of the background presence/absence decisionsection 210, comprising comparators 321, 322, 323 comparing backgrounddensity levels BLr, BLg, BLb calculated with respect to color image dataR, G, B with a predetermined threshold value th1, subtracter 324performing calculation between background density levels BLr and BLg,subtracter 325 performing calculation between background density levelsBLg and BLb, subtracter 326 performing calculation between thebackground density levels BLb and BLr, comparators 327, 328, 329comparing the respective output of the subtracters 324, 325, 326 and apredetermined threshold value th2, AND circuit 330 Anding the respectiveoutputs of the comparators 321, 322, 323, AND circuit 331 Anding therespective outputs of the comparators 327, 328, 329 and OR circuit 332Oring the respective outputs of the AND circuits 330 and 331. Thebackground presence/absence decision section 210 decides whether or notthe background removal be effected in accordance with the followingequations (14) and (15). The background removal is effected whensatisfying

BLr<th1 and BLg<th2 and BLb<th1   (14)

and

|BLb−BLr|<th2

That is, the background removal is done when the background densitylevel BLr, BLg, BLb of image data R, G, B are below a predeterminedfrequency and the level difference of three channels is small.

By the above arrangement, a document whose background is to be removedis decided and the background is removed properly in accordance with thekinds of the documents involved.

Next an explanation will be given below about a seventh embodiment.

The seventh embodiment decides a character/background area from a targetdocument and converts background density on the basis of a result ofdecision.

FIG. 26 shows an arrangement of a major portion of an image processingapparatus according to a seventh embodiment. The seventh embodimentcomprises a density distribution calculation section 201, backgrounddensity distribution calculation section 205, background densityconversion section 206 and character/background decision section 211serving as an area deciding means for deciding a character/backgroundarea. Incidentally, the density distribution calculation section 201,background density distribution calculation 205 and background densityconversion section 206 are the same as those of the above-mentionedthird embodiment with the same reference numerals employed to designatepart and element corresponding to that of the third embodiment and anyfurther explanation omitted.

That is, the character/background decision section 211 decides thecharacter/underground area from the input image data R, G, B, that is, acharacter bearing area in the background. The character/backgrounddecision section 211 decides, for example, an area of a suddenly varyingdensity gradient as a character/background area. The densitydistribution calculation circuit 201 calculates the density distributionof input image data R, G, B with respect to an area decided as thecharacter/background area by the character/background decision section211. The background density distribution calculation section 205calculates the background density distribution of the document on thebasis of the density distribution of the document calculated by thedensity distribution calculation section 201. The background densityconversion section 206 converts the background density of the inputimage data R, G, B on the basis of the background density distributioncalculated by the background density distribution calculation section205.

Next an explanation will be given below about an eighth embodiment.

The eighth embodiment decides a non-photograph area from a targetdocument and converts a background density on the basis of a result ofdecision.

FIG. 27 diagrammatically shows an arrangement of a major section of animage processing apparatus 36 according to an eighth embodiment. Theeighth embodiment comprises a density distribution calculation section201, background density distribution calculation section 205, backgrounddensity conversion section 206 and non-photograph area deciding section212. The density distribution calculation section 201, backgrounddensity distribution calculation section 205 and background densityconversion section 206 are the same as those of the above-mentionedthird embodiment with the same reference numerals employed to designatepart or element corresponding to that of the third embodiment and anyfurther explanation omitted.

That is, the non-photograph area deciding section 212 decides anon-photograph area from the input image data R, G, B. The densitydistribution calculation section 201 calculates a density distributionof input image data R, G, B. The background density distributioncalculation section 205 calculates the background density distributionof the document on the basis of the density distribution calculated bythe density distribution calculation section 201. Based on thebackground density distribution calculated by the background densitydistribution calculation section 205 the background density conversionsection 206 converts the background density with respect to the areadecided as the non-photograph area by the non-photograph area decidingsection 212 with respect to the input image data R, G, B.

Next an explanation will be given below about a ninth embodiment.

The ninth embodiment decides a character/background area from a targetdocument and converts a background density on the basis of a result ofdecision.

FIG. 28 diagrammatically shows an arrangement of a major section of animage processing apparatus according to the ninth embodiment. The ninthembodiment comprises a density distribution calculation section 201,background density distribution calculation circuit 205, backgrounddensity conversion section 206 and character/background deciding section211. It is to be noted that the density distribution calculation section201, background density distribution calculation section 205, backgrounddensity conversion section 206 and character/background deciding section211 are the same as those of the above-mentioned seventh embodiment withthe same reference numerals employed to designate part or elementcorresponding to that of the seventh embodiment and any furtherexplanation omitted.

That is, the character/background deciding section 211 decides thecharacter/background area from input image data R, G, B. The backgrounddensity distribution calculation section 205 calculates the backgrounddensity distribution of the document on the basis of the densitydistribution calculated by the density distribution calculation section201. Based on the background density distribution calculated by thebackground density distribution calculation section 205 the backgrounddensity conversion section 206 converts the background density withrespect to an area decided as the character/background area by thecharacter/background deciding section 211 for input image data R, G, B.

Next an explanation will be given below about a tenth embodiment.

The tenth embodiment decides a character/background area and photographarea from a target document and converts the background density bydifferent methods for the character/background area and photograph area.

FIG. 29 diagrammatically shows an arrangement of a major section of animage processing apparatus according to the tenth embodiment. The tenthembodiment comprises a density distribution calculation section 201,background density calculation section 205, background densityconversion sections 206 a, 206 b, character/background-photographdeciding section 213 and selector 214. The density distributioncalculation section 201 and background density distribution calculationsection 205 are the same as those of the third embodiment with the samereference numerals employed to designate part or element correspondingto that of the third embodiment and any further explanation omitted.

That is, the character/background-photograph deciding section 213decides the character/background area and photograph area from the inputimage data R, G, B. The density distribution calculation section 201calculates the input image data R, G, B. The background densitydistribution calculation section 205 calculates the background densitydistribution of the document based on the density distributioncalculated by the density distribution calculation section 201.

The background density conversion section 206 a converts the backgrounddensity of input image data R, G, B on the basis of the backgrounddensity distribution calculated by the background density distributioncalculation section 205. Based on the background density distributioncalculated by the background density distribution calculation section205 the background conversion section 206b converts background densityof input image data R, G, B by a method different from that of thebackground density conversion section 206 a. The selector 214 selectsthe output of the background density conversion section 206 a when thecharacter/background area is decided by thecharacter/background-photograph deciding section 213 and selects theoutput of the background density conversion section 206 b when thephotograph area is decided by the character/background-photographdeciding section 213.

Next an explanation will be explained below with respect to anembodiment of FIG. 11.

The embodiment of FIG. 11 manually sets a target document as being acolor document or monochrome document and converts background density bya different method relative to the color document or monochromedocument.

FIG. 30 shows an arrangement of a major section of an image processingapparatus 36 according to an eleventh embodiment. The embodiment of FIG.11 comprises a density distribution calculation section 201, backgrounddensity distribution calculation section 205, background densityconversion sections 206 a, 206 b, selector 214 and color/monochromesetting section 215 for manually setting a target document as being acolor document or monochrome document. The density distributioncalculation section 201, background density distribution densitycalculation section 205, background density conversion sections 206 a,206 b and selector 214 are the same as those of the tenth embodimentwith the same reference numerals employed to designate part or elementscorresponding to those of the tenth embodiment and any furtherexplanation omitted.

That is, first, the target document is set as being a color document ormonochrome document by the color/monochrome setting section 215. Thedensity distribution calculation section 201 calculates a densitydistribution of input image data R, G, B. The background densitydistribution calculation section 205 calculates the background densitydistribution of the document on the basis of the density distributioncalculated by the density distribution calculation section 201.

Based on the background density distribution calculated by thebackground density distribution calculation section 205 the backgrounddensity conversion section 206 a converts the background density ofinput image data R, G, B as shown, for example, in FIG. 17. Based on thebackground density distribution calculated by the background densitycalculation section 205 the background density conversion section 206 bconverts the background density of input image data R, G, B, by a methoddifferent from that of the background density conversion section 206 a,as indicated for example in FIG. 10B. The selector 214 selects theoutput of the background density conversion section 206 a when a colordocument is set by the color/monochrome setting section 215 and selectsthe output of the background density conversion section 206 b when amonochrome document is set by the color/monochrome setting section 215.

Next an explanation will be given below about a twelfth embodiment.

The twelfth embodiment automatically decides whether a target documentis a color document or a monochrome document, and converts backgrounddensity in a different method relative to the color document ormonochrome document.

FIG. 31 shows an arrangement of a major section of an image processingapparatus 36 according to the twelfth embodiment. The twelfth embodimentcomprises a density distribution calculation section 201, backgrounddensity distribution calculation section 205, background densityconversion sections 206 a, 206 b, selector 14 and color/monochromedocument deciding section 216 for automatically deciding a targetdocument as being a color document or monochrome document. The densitydistribution calculation section 201, background density distributioncalculation section 205, background density conversion sections 206 a,206 b and selector 214 are the same as those of the eleventh embodimentwith the same reference numerals employed to designate part or elementscorresponding to those of the above-mentioned embodiment and any furtherexplanation omitted.

That is, the color/monochrome document deciding section 216 decides atarget document as being a color document or monochrome document on thebasis of the density difference among the input image data R, G, B as inthe equation (14). The density distribution calculation section 201calculates the density distribution of input image data R, G, B. Thebackground density distribution calculation section 205 calculates thebackground density distribution of the document on the basis of thedensity distribution calculated by the density distribution calculationsection 201.

The background density conversion section 206 a converts the backgrounddensity of input image data R, G, B on the basis of the backgrounddensity distribution calculated by the background density distributioncalculation section 205. Based on the background density distributioncalculated by the background density distribution section 205 thebackground density conversion section 206 b converts the backgrounddensity of input image data R, G, B by a method different from that ofthe background density conversion section 206 a. The selector 214selects the output of the background density conversion section 206 awhen the color document is decided by the color/monochrome documentdeciding section 216 and selects the output of the background densityconversion section 206 b when the monochrome document is decided by thecolor/monochrome setting section 215.

As set out above, according to the present invention, in the case wherea document of a given background is copied, the background density isthinned and a character density is retained. In the case where adocument involving a “back page” emergence is copied, the back image isthinned and a surface image density is retained.

According to the present invention, even if a given document includes aphotograph, a background density at a character area is converted toanother value and the density of a photograph area is faithfullyreserved, and the color and density stay unchanged.

Further, according to the present invention, even if a given documenthas a background color, it is possible to suppress a “back image”emergence while reserving the background color and, at the same time,reduce an uneven shade of the background.

What is claimed is:
 1. An image processing apparatus comprising: densitydistribution calculating means for calculating density distribution of adocument image on the basis of input document image density data, thedensity distribution calculation means having multi-value obtainingmeans for converting input image data to multi-valued image data andhistogram preparing means for preparing a density histogram representingcolor features of the document from the multi-value image data obtainedfrom the multi-value obtaining means, density range calculation meansfor calculating density range corresponding to a background density ofthe document image on the basis of a density distribution calculated bythe density distribution calculating means; and conversion means forconverting the document image density contained in the backgrounddensity range calculated by the density range calculation means toanother density value.
 2. An image processing apparatus comprising:density distribution calculating means for calculating densitydistribution of a document image on the basis of input document imagedensity data, the density distribution calculation means havinghistogram preparing means for preparing a density histogram representingcolor features of the document on the basis of the input image data;density range calculation means for calculating a density rangecorresponding to a background density of the document image on the basisof a density distribution calculated by the density distributioncalculating means, the density range calculation means having means fordetermining a density of a greatest frequency in a low density area ofthe histogram prepared by the histogram preparing means as being abackground density level of the document and calculating the backgrounddensity range on the basis of the background density level; andconversion means for converting the document image density contained inthe background density range calculated by the density range calculationmeans to another density value.
 3. An image processing apparatusaccording to claim 2, characterized in that the conversion means hasmeans for converting input image data below the background density levelcalculated by the density range calculation means to a value “0”.
 4. Animage processing apparatus according to claim 2, characterized in thatthe conversion means has means for converting input image data below thebackground density level calculated by the density range calculationmeans to a value “0” and converting input image data greater than thebackground density level on the basis of a predetermined function.
 5. Animage processing apparatus according to claim 2, characterized in thatthe conversion means has means for converting input image data below thebackground density level calculated by the density range calculationmeans to a predetermined value.
 6. An image processing apparatusaccording to claim 2, characterized in that the conversion means hasmeans for converting only input image data in a predetermined densityrange containing the background density level calculated by the densityrange calculation means to a predetermined level and outputting theother input image directly.
 7. An image processing apparatus accordingto claim 2, characterized in that the density range calculation meanshas means for deciding, as being the background density range, a densityrange near the background density level having a frequency down to afrequency smaller by a predetermined value than a frequency of thebackground density level relative to the image data.
 8. An imageprocessing apparatus according to claim 2, characterized in that thedensity range calculation means has means which, when there is adeviation of an input image data of a predetermined density range withthe background density level as a center, corrects the backgrounddensity level in accordance with the deviation.
 9. An image processingapparatus comprising: density distribution calculating means forcalculating density distribution of a document image on the basis ofinput document image density data; density range calculation means forcalculating a density range corresponding to a background density of thedocument image on the basis of a density distribution calculated by thedensity distribution calculating means; conversion means for convertingthe document image density contained in the background density rangecalculated by the density range calculation means to another densityvalue, and deciding means for deciding whether or not the document is adocument whose background be removed, wherein the conversion meanseffects conversion with respect to only image data of the document whichis decided as the background being removed by the deciding means.
 10. Animage processing apparatus according to claim 9, characterized in thatthe deciding means which, when input image data R, G, B are at abackground level of a predetermined density value and there is a smalldifference among these levels, decides that a background involved beremoved.
 11. An image processing apparatus comprising: densitydistribution calculating means for calculating density distribution of adocument image on the basis of input document image density data;density range calculation means for calculating a density rangecorresponding to a background density of the document image on the basisof a density distribution calculated by the density distributioncalculating means; conversion means for converting the document imagedensity contained in the background density range calculated by thedensity range calculation means to another density value; andcharacter/background deciding means for deciding a background areacontaining characters in input image data, and photograph area decidingmeans for deciding an area as a photograph area in the input image data,wherein the conversion means effects first background density conversionwith respect to an area decided as a background area containingcharacters and effects second background density conversion with respectto the photograph area, the second background density conversiondiffering from the first background density conversion.
 12. An imageprocessing apparatus comprising: density distribution calculating meansfor calculating density distribution of a document image on the basis ofinput document image density data; density range calculation means forcalculating a density range corresponding to a background density of thedocument image on the basis of a density distribution calculated by thedensity distribution calculating means; conversion means for convertingthe document image density contained in the background density rangecalculated by the density range calculation means to another densityvalue; and setting means for setting the document as being a colordocument or a monochrome document, wherein the conversion means effectsfirst background density conversion with respect to a document set bythe setting means as a color document and effects second backgrounddensity conversion with respect to a monochrome document set as being amonochrome document, the second background density conversion differingfrom the first background density conversion.
 13. An image processingapparatus according to claim 12, characterized in that the firstbackground density conversion converts the image density in a backgrounddensity conversion range to a predetermined value and the secondbackground density conversion converts an image density in thebackground density range to a value “0”.
 14. An image processingapparatus comprising: density distribution calculating means forcalculating density distribution of a document image on the basis ofinput document image density data; density range calculation means forcalculating a density range corresponding to a background density of thedocument image on the basis of a density distribution calculated by thedensity distribution calculating means; conversion means for convertingthe document image density contained in the background density rangecalculated by the density range calculation means to another densityvalue; and deciding means for deciding whether the document is a colordocument or a monochrome document, wherein the conversion means effectsfirst background density conversion with respect to a document as beinga color document and second background density conversion with respectto a document as being a monochrome document, the second backgrounddensity conversion differing from the first background densityconversion.
 15. An image processing apparatus according to claim 14,characterized in that the deciding means decides input image data asbeing monochrome document data when there is a small difference amonglevels of the input image data R, G, B and as being color document datawhen otherwise.
 16. An image forming apparatus characterized bycomprising: image reading-out means for reading out a document image andoutputting image data; histogram preparing means for preparing ahistogram representing color features of the document on the basis ofimage data output from the image reading-out means; density rangecalculation means for determining, as a background density level of thedocument, a density of a greatest frequency in a low density area of thehistogram prepared by the histogram preparing means and calculating thebackground density range on the basis of the background density level;conversion means for converting the document image density contained ina background density range calculated by the density range calculationmeans to another density value and outputting it; and image formingmeans for forming an image on the basis of image data provided by theconversion means.
 17. An image forming apparatus according to claim 16,characterized in that means for converting, to a predetermined value,input image data below the background density level calculated by thedensity range calculation means.
 18. An image forming apparatusaccording to claim 16, characterized in that the conversion means has ameans for converting, to a predetermined value, only input image datacontained in a predetermined density range containing the backgrounddensity level calculated by the density range calculation means andoutputting the other input image data directly.